Scanning Electrochemical Microscopy (SECM), one of the major developments in the field of electrochemistry in the past decade, has been shown to be a promising analytical tool for localized studies of surface reactions and their kinetics. In addition, SECM has proved promising for imaging at a nano-scale level. The use of SECM has been demonstrated in a wide range of applications, such as resolving fast heterogeneous kinetics at various material interfaces and imaging of biological molecules. In addition, SECM has been applied in fabrication processes. Studies have shown that metal deposition, metal and semiconductor etching, polymer formation, and other surface modifications with sub micron resolution are feasible when SECM is used.
The achievable localization or spatial resolution of SECM for both analytical and fabrication purposes strongly depends on the shape and size of the electrochemical electrode used. Ultra Micro Electrodes (UME), which are tip probes carrying sub-micron electrodes, are required to obtain resolution at a nanometer scale. Various manufacturing approaches for UMEs have been investigated, ranging from isolation of etched metal wires or Scanning Tunneling Electron Microscopy (STEM) tips for single electrode systems to patch fabrication strategies for electrode array systems.
A combination of SECM with other Scanning Probe Microscopy (SPM) techniques, such as Atomic Force Microscopy (AFM) or Scanning Nearfield Microscopy (SNOM), is highly desirable to obtain complementary surface information simultaneously. In particular, a combination of SECM with the AFM technique can overcome current limitations of SECM, such as uncertainties in distance control of tip to sample. It can additionally allow experiments to study electrochemically initiated changes of topography with simultaneous SECM and AFM.
A crucial component of a combined SECM/AFM system is a specialized probe system, which must be composed of a micro-mechanical bending structure necessary for the AFM mode and an electrochemical UME-tip required for high performance SECM. Several strategies for fabrication of such a probe have been reported.
One strategy is to use modifications of metal wires. An example of this strategy is based on the attachment of a piezo element to the shaft of a conventional UME wire electrode for dithering. The lateral oscillation is measured using an optical laser diffraction measuring system. This type of probe has rarely been used for SECM studies due to the instability of the optical detection system when the tip is largely immersed in a solution. Another example of this strategy is based on shaping cantilever-type SECM tip probes out of a metal wire. Similar to the production of conventional SECM probes, a wire is etched to a pointed tip. Then the wire tip is mechanically bent and flattened to form a cantilever structure. An electrophoretic paint is used as an isolation layer. The disadvantages of this solution include low AFM resolution and mechanical instability of the tip during AFM analysis. A third example of this strategy is to glue a conventional SECM wire-tip to a tuning fork. This way, a commercial NSOM instrument can be used to image surfaces in SECM mode. Disadvantages of this approach include no topographical information and limited spatial resolution.
Another strategy is based on modification of already fabricated AFM cantilever tip probes. The tip and cantilever are metallized to accomplish a conducting bath to the tip. Then all conductive surfaces are insulated except the top area of the tip. A trade off is poor SECM performance mainly because of difficulties with simultaneously providing good tip insulation and useful tip configuration. A similar concept uses FIB technologies to modify AFM cantilever tip probes. Although these efforts resulted in functional tip structures and combined AFM and SECM images were reported, performance issues related to tip sharpness and size of the electrochemical electrode have remained.
The common disadvantage for all of the above described fabrication technologies is a single probe production scheme, which limits miniaturization possibilities and fabrication of multi-probe systems. Accordingly, there is a need in the art to develop methods that allow for production of nano-scale, multi-probe sensors suitable for AFM and SECM analysis.